1. Field of the Invention
The present invention relates to a method for producing a single crystal by growing a semiconductor single crystal such as a silicon single crystal from a melt by the Czochralski method (CZ method), and particularly relates to a method for producing a single crystal, in which setting of a pulling rate of the single crystal from the melt is improved.
2. Description of the Prior Art
In growing the silicon single crystal by CZ method, as publicly known, a seed crystal is dipped in the melt of silicon and the seed crystal is pulled up in this state while controlling a pulling rate to obtain a cylindrical silicon single crystal on a lower end of the seed crystal. Then, a silicon wafer, which is to be used as a material for a semiconductor device, is taken from the silicon single crystal grown.
It has been known that the pulling rate of the silicon single crystal, in other words, the pulling rate of a body portion has a large effect on the defect distribution in a radial direction of the silicon single crystal. This means that in the silicon wafer, when subjected to thermal oxidation treatment, oxidation induced stacking faults called OSF may occur in a form of a ring. A region (hereafter, this region is referred to as an OSF ring-occurring region) where this OSF may occur is known to move toward the periphery with increase in pulling rate.
While the OSF ring is a kind of thermal treatment induced defect, infrared scattering defect and dislocation cluster formed during crystal growth are called grown-in defects. Among grown-in defects, infrared scattering defects occur inside the OSF ring and degrade the gate oxide integrity. On the other hand, dislocation clusters occur outside the OSF ring appearing on both sides of a non defect region.
With a change of a diameter of the region where the OSF ring occurs due to a change of the pulling rate, the area of the region, where these grown-in defects occur, changes. Specifically, a high pulling rate will increase the diameter of the region where the OSF ring occurs and expand the area inside this region, where these grown-in defects occur. A lower pulling rate will decrease the diameter of the region where the OSF ring occurs and reduce the area inside this region, where these grown-in defects occur.
In recent years, with a lower room temperature and a lower content of oxygen in the single crystal in semiconductor producing processes, an adverse effect of the OSF on a device is being suppressed. Therefore, it has become important to reduce the density of grown-in defects, namely, infrared scattering defects occurred inside the OSF ring-occurring region, which have an adverse effect on gate oxide integrity Therefore, it is proposed to set a pulling rate such that the density of grown-in defects occurred inside the OSF ring-occurring region may be reduced.
It has been well known that the pulling rate of the silicon single crystal has also a large effect on a shape of the single crystal section. Specifically, the crystal deformation (ellipticity) expressed by (maximum diameter-minimum diameter)/minimum diameter of the same section of the single crystal increases with increase in pulling rate. When the crystal deformation increases, a product portion taken from the grown crystal decreases to lower the yield. Therefore, it is also practiced to set the pulling rate to make the crystal deformation fall within a predetermined range.
With respect to such setting of the pulling rate of the silicon single crystal, the following prior art has been disclosed in Japanese Patent Laid-Open No. 11-189489. According to Japanese Patent Laid-Open No. 11-189489, a technique for reducing the density of the grown-in defects is disclosed, wherein a profile of the pulling rate is determined in advance such that the crystal deformation is maintained within the range of from 1.5 to 2.0 percent over the full length from a top to a bottom of the body portion in the direction of pulling axis, and the profile of the pulling rate multiplied by xcex1 (xe2x89xa60.8) is used as an aimed profile of the pulling rate in actual crystal.
Here, xcex1 is a coefficient for setting the aimed value to the diameter (defect region diameter D) of the region, where grown-in defects occur, located inside the OSF ring-occurring region, and is calculated from the change rate of the defect region diameter D.
In growing the silicon single crystal, growing conditions change, for example, an amount of melt remained decreases and a heater power changes with the progress of growth and thus, if the pulling rate is maintained constant throughout the entire period of pulling, quality is likely to be deteriorated in later stages of pulling. In addition, for either the defect distribution in the crystal radial direction or the crystal deformation, the pulling rate must be gradually lowered with the progress of growth in order to yield the same profile in the full length of pulling axis direction of the body portion.
According to the prior art disclosed in the Japanese Patent Laid-Open No. 11-189489, for the following reasons, it has been described that reduction of density of grown-in defects is possible.
The crystal deformation increases with increase in crystal-pulling rate. Maintaining the crystal deformation to such relatively high range of from 1.5 to 2.0 percent fixes the OSF ring-occurring region to the outer circumferential portion of the crystal. This is used as a standard profile of the pulling rate and this value multiplied by xcex1 (xe2x89xa60.8) is used as the aimed profile of the pulling rate in actual crystal growth. Thus, the defect region diameter D is determined in accordance with a. When the a is selected from the range equal to or smaller than 0.8, the defect region diameter D is minimized; and the non defect region occurring in adjacent region outside the OSF ring-occurring region is effectively used. As a result, density of grown-in defects is reduced,
Though the crystal deformation and the change rate of the defect region diameter D can be actually used as an index in setting the pulling rate, in practice of crystal growth, these are insufficient For example, when the change rate of the defect region diameter D is small, even if the crystal deformation is maintained in the range of from 1.5 to 2.0 percent, a set value of the pulling rate possibly increases unlimitedly. This is because when the crystal deformation is smaller than the aimed value of 1.5 to 2.0 percent, some pulling rate is always added to the aimed pulling rate profile when the pulling rate becomes too high, a stable operation cannot be performed and a situation is expected in which density of grown-in defects exceeds an assumed value.
The object of the present invention is to provide a method for producing a single crystal, allowing for minimizing density of grown-in defects as well as stable practical operation.
To achieve the above described purpose, according to this invention, a method for producing a single crystal by growing a semiconductor single crystal from a melt by CZ method is provided, wherein as an index of deformation of the single crystal from a perfect circle, an aimed value dAIM of a crystal deformation defined by (maximum diameter-minimum diameter)/minimum diameter of a crystal section is previously calculated; an upper limit value VMAX of a pulling rate necessary for suppressing a defect density to an allowable range is calculated from distribution of grown-in defect in the crystal section, the single crystal is pulled up according to a predetermined pulling rate, then deviation xcex94d of the achieved value dACT of the crystal deformation from said aimed value dAIM in pulling is calculated, the deviation xcex94d is converted to a correction xcex94V of the pulling rate, this correction xcex94V is added to a set value of the pulling rate in said pulling; the pulling rate obtained by this addition is compared with said upper limit value VMAX and a smaller pulling rate is determined as the set value of the pulling rate in the next and following runs.
According to the method for producing the single crystal of the present invention, the crystal deformation is controlled to maintain the aimed value dAIM thereof. Density of grown-in defects in the crystal section is suppressed to the allowable range. Satisfying these conditions, pulling is carried out in a pulling rate as fast as possible. In addition, in CZ process, product specifications, such as the crystal deformation are sensitively influenced by a difference in thermal conditions in a furnace and sensitivity is very high and thus, change in pulling rate causes a big effect. However, because the value of the pulling rate added by the correction xcex94V is compared with the upper limit value VMAX thereof, and the smaller value is used as the set value of the pulling rate in the following runs, an abrupt change of pulling rate can be avoided resulting in a more stable operation.
The aimed value dAIM of the crystal deformation can be the value almost corresponding to the upper limit of deformation from the perfect circle of the section of the single crystal. Consequently, deformation can be suppressed to the allowable range to enable faster pulling.
The correction xcex94V (mm/min) of pulling rate can be calculated by the following equation. For this calculation a gain N is introduced and thus, the abrupt change of pulling rate can be avoided resulting in a more stable operation.
xcex94V=Nxc3x97efxc3x97xcex94d
N: gain being the value (xe2x88x92) falling within the range of 0.1 to 1.0
ef: effect coefficient [(m/min)/percent]
xcex94d: deviation (%) of crystal deformation
For the correction xcex94V of the pulling rate, a calculated value of the correction xcex94V of the pulling rate is compared with the allowable upper limit thereof and if the calculated value exceeds the allowable upper limit, the allowable upper limit can be used as the correction xcex94V of the pulling rate. Consequently, an abrupt change of pulling rate can be avoided resulting in a more stable operation.
The pulling rate can he set at a plurality of points with the transition of the pulling length and the pulling rate in each point can be set to decrease monotonously according to the progress of pulling. Consequently, fluctuations of the crystal pulled in the later stage of pulling can be suppressed to avoid quality deterioration. Even if the pulling rate is monotonously decreased with the progress of pulling, the diameter of the crystal can be kept constant by regulation of the heater power and the like.
The crystal deformation can be smoothed on the basis of achievement of a plurality of pulling runs to use it in setting the pulling rate for the following runs. Consequently, an abrupt change of pulling rate can be avoided resulting in a more stable operation. As a method for smoothing, calculating a simple average, employing an weighted average, and employing a median can be employed.